Lighting arrangements that distribute power density over a long rectangular region are used for high speed sorting and testing of various products, including foodstuffs, vegetables, paper, pharmaceauticals, plastics, recyclables, and the like. These systems illuminate a moving sheet of product so that the product may be inspected with cameras or spectroscopic instruments (see, for example: http://www.satake-usa.com, http://www.bestnv.com/, and http://www.buhlergroup.com/17217EN.asp, http://www.magsep.com/).
In order to create adequate lighting for such inspection devices, the spatial distribution of optical power preferably satisfies several requirements with respect to power density, angular distribution of optical power and wavelength dependence and efficiency. Power density (e.g., mw per cm2) is preferably uniform within a few percent inside the entire light field (i.e., the rectangular region), which often measures (for example) 5 mm by 1 meter and may also fall to low levels outside the rectangle. The angular distribution of optical power preferably allows the power density requirements to be met with working distances (e.g., several cm up to and greater than a meter) compatible with typical sorting machines. With regard to wavelength dependence and efficiency, these parameters preferably are compatible with practical light generation devices.
Prior solutions of providing lighting for high speed sorting equipments include the concepts illustrated in FIGS. 1A-1F. For the sake of clarity, optical lensing elements that shape the beam in one or two axes have been deleted. Moreover, each figure includes reference numeral 100 which represents the illuminated area and rays of light are depicted as arrows in each of the figures.
For example, in FIG. 1A, an illuminated area 100 is provided by a system which includes tube lamps and infrared filaments 102. While the sources for such a system are low cost, they suffer from low intensity, poor directionality, poor efficiency and thermal problems. FIG. 1B illustrates an array of bright LEDs 104 used to create a line source. However, since such a large number of sources are required to create a uniform power density, the cost and complication of the resulting system is typically prohibitive.
FIGS. 1C and 1D illustrate systems which both utilize a single light source. The system illustrated in FIG. 1C uses a bulk arrangement with multiple lenses 108 and light beam splitters 110, which uses a single light source 106, while the system in FIG. 1D accomplishes the function with an optical fiber bundle 114 which is coupled to light source 112 (see U.S. Pat. No. 4,730,895).
FIG. 1E illustrates the use of a grooved light guide for creating a continuous, uniform line source supplied by one light generating device. Specifically, light from light source 116 is transmitted down a lightguide 118 that includes a surface relief 120 on one sidewall. Light scatters from the surface relief and is directed out of the guide toward the illuminated target. This approach has been addressed in the following ways:                holographic scatter centers written on the back sidewall (U.S. Pat. No. 5,721,795);        prismatic scatter centers placed on a lens in the emission surface (U.S. Pat. No. 5,671,306);        facets placed on the back sidewall (U.S. Pat. No. 5,894,539); and        facets placed on a 2D surface for LCD illumination (U.S. Pat. No. 6,036,327).While these scatter based methods can create uniform illumination, each system is generally inefficient and the resulting light field lacks directionality.        
FIG. 1F illustrates a system which includes a collimated laser beam from light source 122 is rapidly raster scanned using a rotatable mirror element 124 over a target to create uniform time-averaged illumination. This approach is costly and exposes operators to free space laser power (U.S. Pat. No. 6,864,970).
Another way to create a line source is to make a side-emitting fiber. Such fibers emit light in all directions by either using “leaky” modes or by placing randomly positioned scatter centers in the core (see Optics and Photoncs News, vol 16, No 10, 2005 and U.S. Pat. No. 6,546,174 and U.S. Pat. No. 6,347,172, all of the foregoing herein incorporated by reference in their entirety). These side-emitters are used for architectural lighting, and are commercially available (see, for example, http://www.svision.com/sign.html# and http://www.laselite.com/).
Accordingly, it is recognized that it would be advantageous to develop a lighting arrangement which solves the above noted drawbacks, preferably distributes significant power density over a shaped field/region, and which is cost effective.